Q. I have attempted to grow watermelons for the first time last summer and everything was looking good until it got hot. These tiny black specks appeared on the back of some of the leaves. I was told it was aphids but they weren’t sure. I hope you can see them clearly from the picture I've attached. Should I use insecticidal soap as recommended or something else?
A. I tried to magnify the picture so I could see the black spots better. I was curious if these were in fact from insects or not. I could not see it clearly but whatever it was, it appeared that it was no longer a problem and the leaves seemed nice and healthy.
Aphids are cool weather insects and so when the heat hit they should have disappeared. The big insect problems for you on watermelon would be aphids earlier in the season during cool weather, squash bugs shortly after planting and whiteflies (during the heat).
Yellow sticky traps can provide a method to determine if whiteflies are a problem or not. I will post a short video on my blog on how to make these rather than buy them. They are handy to place in the garden for insect monitoring.
Soap and water sprays are good but must be done about every three or four days and the underside of the leaves must be sprayed as well. Squash bugs are nasty and can be vacuumed or hand-picked or delay planting by seed until after June 1. If you are hand picking or vacuuming squash bugs then be diligent for about three or four weeks and keeping the plants cleaned off every couple of days by hand until mid-June to avoid most of the problems with this insect.
Thanks to Garry Pearson, Lead Greenhouse Managerat UC Davis CAES,
Department of Vegetable Crops, during his presentations on greenhouses in Afghanistan.